The 17 Mdal pAMbeta1 plasmid is known to confer erythromycin resistance in the streptococci. It is closely related to an apparently large family of such plasmids which are widely disseminated in several streptococcal Lancefield groups. pAMbeta1 may be readily introduced into Streptococcus sanguis via genetic transformation by direct selection for erythromycin resistance. S. sanguis is an exopolysaccharide-producing organism closely related to the cariogenic Streptococcus mutans in its ability to form adherent deposits (plaque) on smooth tooth surfaces. We have isolated a derivative of the pAMbeta1 plasmid which is 7.3 Mdal in size. The apparent deletion event which resulted in the formation of the 7.3 Mdal plasmid (desingated pZA1) occurred spontaneously in S. sanguis. pZA1 is cut once with a specific restriction endonuclease which yields single stranded overlapping ends. This plasmid thus is being tested as a vector in a "self-cloning" S. sanguis system uisng standard recombinant DNA methods. Derivatives of pZA1 are being constructed by in vitro shortening of the plasmid molecule in order to develop improved cloning vectors in this system. All potential cloning vectors are being thoroughly characterized with respect to size, copy number, restriction nuclease sites, and ability to be amplified. These plasmid vectors will be used to develop a clone bank of the S. sanguis chromosome in a S. sanguis host. In this fashion, we hope to be able to locate and enumerate the genes involved in exopolysaccharide synthesis. These studies will contribute greatly to our understanding of the biochemistry and genetics of oral colonization by exopolysaccharide producing streptococci.